VOLCANOSTRATIGRAPHY OF THE DIVISADERO TUFF IN RIO SAN IGNACIO CANYON, CREEL, MEXICO

The Sierra Madre Occidental Volcanic Field in northwestern Mexico is one of the largest ignimbrite provinces in the world yet much of it has been mapped only at the reconnaissance level. The local Jesuit Mission in Creel, Chihuahua, Mexico, has been drilling wells for Tarahumaran Indian villages situated in the volcanic field, but many wells have been unproductive due to a lack of knowledge of the hydrogeology. Utah Valley University students and faculty traveled to Creel, in October 2009 as part of an ongoing hydrogeologic study to aid the Mission's well-drilling program. The area in which the mission's well drilling program is focused has previously only been mapped as the undifferentiated 29 Ma Divisadero Tuff. Our work in the area suggests that welded tuffs at the bases of cooling units within the Divisadero Tuff are susceptible to fracturing and are better potential aquifers than unwelded tuffs. Consequently, we are working to document the volcanostratigraphy of the Divisadero Tuff. Herein we report on a section through the Divisadero Tuff along the road to Rekowata Hot Springs in the Rio San Ignacio canyon, which has over 350 m relief. Two distinct flat-lying cooling units were identified, along with a possible third. Each cooling unit exhibits a transition from moderately welded, pumice rich tuff, found in the upper sections, to a strongly welded tuff with highly flattened pumice in the lower portions. The stratigraphically lowest identified cooling unit has a thickness of ~187 m, the upper cooling unit has a thickness of ~160 m. The cooling units are separated by clastic sedimentary strata about 30 m in thickness, indicating a significant time period between eruptions. In the nearby village of Basigorabo we identified a welded tuff that overlies a moderately welded tuff and is stratigraphically above the two units in the canyon. The welded tuff in Basigorabo is probably the base of a third, younger cooling unit and is under further study by other members of our group.